1. Field of Disclosed Subject Matter
This disclosure relates to systems and methods that incorporate a dual layer plate configuration including at least a robust top imaging layer and a bottom layer that acts as a reservoir for the releasing oil in a proposed variable digital offset lithographic architecture.
2. Related Art
Lithography is a common method of printing or marking images on an image receiving medium. In a typical lithographic process, the surface of a print image carrier, which may be a flat plate, cylinder or belt, is formed to have “image regions” of hydrophobic and oleophilic material, and “non-image regions” of a hydrophilic material. The image regions correspond to the areas on the final print on the image receiving medium that are occupied by a printing or marking material such as ink. The non-image regions are the regions corresponding to the areas on the final print on the image receiving medium that are not occupied by the printing or marking material. The hydrophilic regions accept, and are readily wetted by, a water-based dampening fluid, which is commonly referred to, and will be generally referred to in this disclosure, as a “fountain solution.” The fountain solution typically consists of water and a small amount of alcohol, and may include other additives and/or surfactants that facilitate non-adherence of ink in those regions. The deposition of fountain solution over the hydrophilic regions forms a fluid “blocking layer” for rejecting ink. Therefore, the hydrophilic regions of the printing plate correspond to unprinted areas, or “non-image areas”, of the final print on the image receiving medium. The hydrophobic regions repel the fountain solution and accept the ink.
Depending on a configuration of a conventional lithography system, the ink may be transferred directly to a substrate of image receiving media, such as paper, or may be applied to an intermediate surface, such as an “offset” (or blanket) cylinder. This latter configuration is referred to as an offset lithographic printing system. The offset or blanket cylinder is covered with a conformable coating or sleeve with a surface that can conform to the texture of the image receiving medium substrate, which may have surface peak-to-valley depth somewhat greater than the surface peak-to-valley depth of the imaging plate. Sufficient pressure is used to transfer the image from an imaging member or from the offset or blanket cylinder to the substrate. The substrate of image receiving media is pinched between the imaging member or the offset or blanket cylinder and an impression cylinder that provides pressure against the imaging member or the offset or blanket cylinder to provide an imaging nip through which the substrate of image receiving media passes to have the image printed or marked thereon.
Conventional lithographic and offset lithographic printing techniques use plates that are permanently patterned, and are, therefore, generally considered to be useful only when printing a large number of copies of the same image in long print runs, such as for magazines, newspapers, and the like. These conventional processes are generally not considered amenable to creating and printing a new pattern from one page to the next because, according to known methods, removing and replacing of plates, including on a print cylinder, would be required in order to change images. For these reasons, conventional lithographic techniques cannot accommodate true high speed variable data printing processes in which the images to be printed change from impression to impression, for example, as in the case of digital printing systems. Additionally, the cost of the permanently-patterned imaging plates or cylinders is amortized over the number of copies of a document that are produced. The cost per printed copy is, therefore, higher for shorter print runs of the same image than for longer print runs of the same image, as opposed to prints from variable data digital printing systems.
The lithography process provides very high quality printing at least in part due to the quality and color gamut of the inks used. The inks, which typically have a very high color pigment content, typically in a range of 20-70% by weight, are very low cost compared to toners and many other types of printing or marking materials. This comparatively low cost generates a desire to use the lithographic and offset inks for printing or marking in order to take advantage of the high quality and low cost in a manageable manner if a system can be made manageable to printing variable image data from page to page. Previously, the number of hurdles to providing variable data printing using lithographic inks appeared insurmountable. Even the desire to reduce a cost per copy for shorter print runs of the same image presented challenges. Ideally, the desire is to incur the same low cost per copy of a long offset or lithographic print run, e.g., of more than 100,000 copies, for a medium print run, e.g., on the order of 10,000 copies, and for a short print run, e.g., on the order of 1,000 copies. Full implementation of a variable printing scheme using lithographic inks may ultimately result in the economies reaching the single copy print run in a true variable data printing system or method.
Efforts have been made to create lithographic and offset lithographic printing systems for variable data in the past. One example is disclosed in U.S. Pat. No. 3,800,699 in which an intense energy source such as a laser is used to pattern-wise evaporate a dampening fluid. In another example disclosed in U.S. Pat. No. 7,191,705, a hydrophilic coating is applied to an imaging belt. A laser selectively heats and evaporates or decomposes regions of the hydrophilic coating. A water-based fountain solution is then applied to these hydrophilic regions, rendering them oleophobic. Ink is then applied and selectively transferred onto the plate only in the areas not covered by fountain solution, creating an inked pattern that can be transferred to a substrate of image receiving media. Once transferred, the imaging belt is cleaned, a new hydrophilic coating and fountain solution are deposited, and the patterning, inking, and printing steps are repeated, for example, for printing the next batch of images.
In yet another example, a rewritable surface is used that can switch from hydrophilic to hydrophobic states without application of thermal, electrical or optical energy. Examples of these surfaces include the so-called switchable polymers and metal oxides such as ZnO2 and TiO2. After changing the surface state, fountain solution selectively wets the hydrophilic areas of the programmable surface and, therefore, causes a rejection of the application of ink to these areas. These switchable coatings, particularly the switchable polymers, tend to be expensive to coat onto a surface and are typically prone to excessive wear. Also, these switchable coatings tend not to have the capacity to transform between hydrophobic and hydrophilic states in the sub-millisecond time range that would be required to enable high-speed variable data printing using lithographic techniques. Based on this, the effectiveness of using switchable coatings may be in limited short-run print projects rather than being adaptable to truly variable data high-speed digital lithography in which every impression can have a different image pattern changing from one print cycle to the next.
The above-described attempts at implementing variable data lithographic printing still suffered from numerous difficulties. For example, most imaging plate or belt surfaces using lithographic printing have a micro-roughened surface structure to retain fountain solution in the non-imaging areas. The micro-roughened surface aids in retaining the liquid fountain solution, enhancing an affinity toward the fountain solution so that the liquid does not get forced away from the surface by, for example, action at a roller nip. Shearing forces in the nip between the imaging surface and the ink forming roller can overwhelm any static or dynamic surface energy forces drawing fountain solution to the surface.
A difficulty arises, however, in that these micro-roughened surfaces are difficult to clean by conventional mechanical means such as, for example, by using knife-edge cleaning systems for scraping residual ink from the plate or belt surface. The knife simply cannot get into the pits in the micro-roughened surface, which is there to effectively retain the fountain solution. Additionally, physical contact between the knife and the plate or belt surface results in significant wear. Once the surface is worn, there is a relatively high cost of replacing a plate or belt. Non-contact cleaning processes, such as high-pressure rinsing or solvent cleaning are possible. These cleaning processes, however, tend to increase costs significantly, not only based on the inclusion of required additional subsystems, but also on a potential cost associated with hazardous waste disposal. Further, to date, these non-contact cleaning processes are of unproven effectiveness.
In an effort to improve cleaning on each pass, with an objective of providing ghost-free printing, the prior art systems describe using a very smooth belt or plate surface. See, e.g., U.S. Pat. No. 7,191,705 referenced above. Known techniques for cleaning the surface are more effective on these smooth surfaces. Physical scraping still has an effect of wearing the physical surface, but it is lessened. The difficulty with using smooth surfaces is that the advantage in being able to clean the smooth surface is offset with the reduced ability to retain a hydrophilic coating and printing or marking material as compared to the micro-roughened surface. So surfaces, therefore, may necessitate employing additional and costly subsystems such as, for example, surface energy conditioning subsystems including a corona discharge apparatus, which themselves can induce wear or damage to the plate or belt surface. Precise metering of the fountain solution additionally can become more difficult without the presence of correct texture such as, for example, with the micro-roughened surface. Also, spreading or other lateral movement of the fountain solution on a texture-free surface may compromise ultimate imaging resolution.
Another disadvantage encountered in attempting to modify conventional lithographic systems for variable printing is a relatively low transfer efficiency of the inks off of the imaging plate or belt. Common conventional lithographic and offset printing or marking processes operate with ink transfer ratios on the order of approximately 50-50, i.e., about half of the ink that is applied to the “reimageable” surface actually transfers to the image receiving media substrate requiring that the other half of the ink be cleaned off the surface of the plate or belt and removed. This relatively low efficiency compounds the cleaning problem in that a significant amount of cleaning is required to completely wipe the surface of the plate or belt clean of ink so as to avoid ghosting of one image onto another in variable data printing using a modification of conventional lithographic techniques. Also, unless the ink can be recycled without contamination, the effective cost of the ink is doubled. Traditionally, however, it is very difficult to recycle the highly viscous ink, thereby increasing the effective cost of printing and adding costs associated with ink disposal. Proposed systems fall short in providing sufficiently high transfer ratios to reduce ink waste and the associated costs. A balance must therefore be struck in the composition of the ink to provide optimum spreading on a plate or belt surface including adequate separation between printing and non-printing areas, increased ability to transfer to a substrate, and an ability to clean the ink in a manner that produces less wear on the plate or belt.